1. Field of the Invention
The invention relates to memory devices and more particularly to changing the operating mode of memory units. In this context, it is possible to change between a low-power operating mode and a normal-power operating mode.
2. Description of the Related Art
To reduce the power consumption of electronic appliances, memory units are normally changed to a low-power operating mode during time periods in which they are not required. Depending on the type of memory unit, the low-power operating mode may be what is known as the data retention mode in the case of SRAMs (static random access memory) or the self-refresh mode in the case of DRAMs (dynamic random access memory) or isolation of the supply voltage in the case of ROMs (read-only memory).
In general terms, a low-power operating mode in the present patent application is understood to mean an operating mode in which the power requirement of the relevant component or of the relevant circuit arrangement is reduced and accordingly the component or the circuit arrangement cannot execute the functions for which it is actually designed during normal operation. By way of example, a standby mode is a low-power operating mode.
In a normal-power operating mode, the functions of the relevant component or of the relevant circuit arrangement for which the component and the circuit arrangement are designed can be executed. By way of example, in the normal-power operating mode it is possible to access a memory unit, i.e. data values can be stored in the memory unit and data values can be read.
To reduce the power consumption of circuit arrangements with memory units as far as possible, the memory units need to be changed to suitable low-power operating modes as often as possible. Different operating modes of the circuit arrangement give rise to different demands on the memory units and their operating modes. Added to this is the fact that the operating modes of the circuit arrangement can change dynamically during operation.
In conventional circuit arrangements, the operating mode of the associated memory units is controlled and configured by a piece of software. In this context, the operating mode can be configured statically or dynamically.
The text below cites a few examples for setting the operating mode statically. Thus, hardware units in a memory system can be set by means of a piece of software such that they isolate a ROM, which is required for booting the system, from the supply voltage after a boot operation has taken place. In addition, a memory unit can be set by a piece of software after a reset in accordance with the current configuration of the superordinate system. An example of this is a mobile radio which has been switched off and which can be started up only in a battery charging mode.
An example of the dynamic configuration of the low-power operating mode is a piece of hardware which is configured by a piece of software, during a time period in which it is not necessary to access a particular ROM memory unit, such that the hardware isolates the ROM memory unit from the supply voltage. In addition, by way of example, an SDRAM controller can be prompted by a piece of software to change a DRAM to a self-refresh mode while the superordinate system is in an idle state.
A drawback of systems in which a piece of software sets the operating mode of memory units is that the software needs to know precisely in what operating state of the circuit system the relevant memory units are needed and not needed. The result of this is increased complexity of the software, with the software-based dynamic configuration of the operating mode having an even more disadvantageous effect on the complexity of the software than the static configuration.
Another problem of software-based configuration of the operating mode of memory units is that the software must explicitly control the changes between the various operating modes. Consequently, the software needs to be informed about what content of the various memory units and their subunits is needed in the various operating modes of the circuit system. In addition, the software also needs to know what volatile memory contents must not be lost during a low-power operating mode because they are needed again as soon as the circuit system is in another operating state. It is therefore necessary to ensure that these data are retained.
As a result, this means that conventional circuit arrangements need to have a very complex and hence expensive piece of software in order to be able to make changes of operating mode for their memory units.